Fluorosilicone elastomers have long been known for their solvent resistance, flexibility at low temperatures and excellent high temperature properties. Many industrial and military applications have been developed which exploit these desirable characteristics (e.g., aircraft sealant in view of good resistance to jet fuel). A fluorosilicone-fluorocarbon hybrid system described by Pierce et al. (Applied Polymer Symposium, No. 22, 103-125, 1973) is stated to be particularly resistant to reversion at elevated temperatures, the repeat unit of such a hybrid polymer being exemplified by the formula --RR SiCH.sub.2 CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 SiRR'--wherein R is a methyl radical and R' is a trifluoropropyl radical. This hybrid polymer can be prepared by reacting a terminally unsaturated intermediate of the formula CH.sub.2 .dbd.CHCF.sub.2 CF.sub.2 CH.dbd.CH.sub.2 with a chlorosilane of the formula RR'SiHCl in the presence of a platinum catalyst, followed by hydrolysis and condensation of the resulting chlorine-terminated monomer. The terminally unsaturated intermediate, in turn, is prepared by the following series of steps: ##STR1## wherein the synthesis of dibromoperfluoroalkylalkanes (II) and (III) each takes place in the presence of a catalytic amount of an organic free-radical generator. Pierce et al, noted that, while the formation of compound (II) according to the first of these reactions is relatively facile, only "trace" amounts of the desired compound (III) result according to the second reaction wherein about 3 mole percent of an organic peroxide was used as the catalyst. This conclusion was also reported by Kim et al, in a previous paper (J. Fluorine Chem., 1, 203-218, 1971).
Although it is possible to produce small yields of telomer (III) directly from compound (I), this is not desirable. Under these conditions, it has been observed that a large excess of ethylene must be used and significant quantities of unwanted telomers of the type BrCF.sub.2 CF.sub.2 (CH.sub.2 CH.sub.2).sub.j Br, in which j is an integer having a value of 2 to 10, are formed as byproducts. Therefore, the above describe sequence, wherein (I) is reacted with a deficiency of ethylene to form (II), the latter is isolated and subsequently further reacted with ethylene to form telomer (III), is the preferred route.
Reactions similar to those discussed by Pierce et al, and Kim et al., cited supra, are disclosed in U.S. Pat. No. 3,055,953 to Smeltz. Here, compounds of the type Br(CH.sub.2 CH.sub.2).sub.m (CF.sub.2 CF.sub.2).sub.n (CH.sub.2 CH.sub.2).sub.p Br and Br(CF.sub.2 CF.sub.2).sub.n (CH.sub.2 CH.sub.2).sub.p Br, in which m and p are integers in the range 1 to 6 and n is an integer in the range of 1 to 10, are prepared by reacting Br(CF.sub.2 CF.sub.2).sub.n Br or Br(CF.sub.2 CF.sub.2).sub.n (CH.sub.2 CH.sub.2).sub.p Br with ethylene. These reactions are carried out at superatmospheric pressures at 50.degree. C. to 200.degree. C. in the presence of a free-radical generating catalyst. Starting pressures of at least 150 psi (1,034 kPa) are used and the molar ratio of peroxide catalyst to the bromofluorocarbon reactant was no greater than 0.043 in any of the examples. Example 5 of the Smeltz patent explicitly discloses the high pressure reaction of Br(CF.sub.2 CF.sub.2).sub.2 CH.sub.2 CH.sub.2 Br with ethylene using a catalytic amount of t-butyl peroxide (i.e., peroxide/dibromide reactant in a molar ratio of 0.02/0.46=0.043. However, this reference does not specifically teach the corresponding reaction of BrCF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 Br (II) with ethylene to produce the highly desirable product BrCH.sub.2 CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 Br (III).
Although the above references suggest that compounds of the type BrCH.sub.2 CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 Br can be prepared by the free-radical catalyzed telomerization of ethylene with a telogen of the type BrCF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 Br, the yield of the desired telomer product is relatively small, and these reactions are therefore more of academic than practical interest. Moreover, when the present inventor employed the methods of Kim et al. to react BrCF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 Br with ethylene in the presence of t-butyl peroxide, the conversion of the telogen and yield of BrCH.sub.2 CH.sub.2 CF.sub.2 CF.sub.2 CH.sub.2 CH.sub.2 Br were even lower than the corresponding values reported by Kim et al. It is believed that these lower values can be attributed, at least in part, to a more careful workup (i.e., distillation, analysis and material balance) of the reaction product mixture than was undertaken by Kim et al. There is therefore still a need for an improved process for synthesizing the highly desirable telomers described supra.